The technical field of this invention is laser ablation of surfaces, especially surfaces of biological materials. In particular, the invention relates to systems and methods for reprofiling the cornea of the eye.
The cornea comprises transparent avascular tissue that forms the anterior portion of the eye. The cornea functions as both a protective membrane and as a "window" through which light passes as it proceeds to the retina. The transparency of the cornea is due to its uniform structure, avascularity, and deturgescence, which is the state of relative hydration of the corneal tissue. The average adult cornea is about 0.65 mm thick at the periphery, and about 0.54 mm thick in the center. From anterior to posterior, the cornea has the following five distinct layers: the epithelium, Bowman's membrane, the stroma, Descemet's membrane, and the endothelium. The present invention concerns the epithelium, Bowman's membrane, and the stroma. The epithelium consists of five or six layers of cells, and the underlying Bowman's membrane, a clear acellular layer, is a modified portion of the stroma. The corneal stroma accounts for about 90 percent of the corneal thickness.
Various techniques have been proposed for shaping the cornea of a patient's eye to treat, e.g., myopia. The general technique involves removing the epithelium layer, and then shaping the underlying Bowman's and stroma layers, either surgically, or by using photoablation.
One technique, described in the Muller, U.S. Pat. No. 4,856,513 (assigned to the present assignee), and incorporated herein by reference, uses a laser and an erodible mask with a predefined profile of resistance to erosion by laser radiation which is disposed between the laser and the corneal surface. A portion of the laser radiation is absorbed by the mask, while another portion is transmitted to the corneal surface in accordance with the mask profile, thereby selectively photoablating the corneal surface into a desired shape.
In another technique, described in Marshall et al., U.S. Pat. No. 4,941,093 (assigned to the present assignee), also incorporated herein by reference, the shape and size of the area of the corneal surface irradiated by laser energy is selected and controlled so that some areas of the corneal surface become more eroded than others and a desired corneal shape is achieved.
Other techniques for corneal reprofiling, or photorefractive keratectomy ("PRK"), are also disclosed in L'Esperance, U.S. Pat. No. 4,773,414; Muller, U.S. Pat. 5,019,074; Trokel, U.S. Pat. 5,108,338; and Munnerlyn, U.S. Pat. 5,163,934, likewise incorporated herein by reference.
Typically, in PRK operations, the epithelium layer is removed mechanically prior to shaping the cornea with photoablation. Generally, the ophthalmologist removes an area of epithelium that is about 2 mm in diameter larger than the area that is to be shaped. It is known that the stroma is the corneal layer that must be shaped in corrective surgical procedures of the eye. By removing the epithelium prior to surgery, the required corrective steps can be performed directly on the stroma.
In fact, the prior art references have consistently taught the desirability of mechanically removing the epithelium prior initiation as laser ablation. For example, in U.S. Pat. 4,773,414, L'Esperance states that it is important to remove the epithelium locally from the interior surface of the cornea "as to assure no ultraviolet irradiation of the epithelium." Similarly, in U.S. Pat. No. 4,940,093, Marshall states that "the overlying epithelium of the cornea must be removed prior to reprofiling . . . " In addition, in U.S. Pat. 5,163,934, Munnerlyn states "photodecomposition to ablate away corneal tissue occurs, according to my invention, only after the epithelium is surgically removed. Essentially, the corneal epithelium until now has been regarded primarily as a nuisance that only degrades the accuracy of the corneal reprofiling unless it is first removed.
Once the epithelium is removed, laser reprofiling of the stroma can be commenced in accordance with any of the above-referenced prior art techniques. Following the corneal shaping procedure, the epithelial layer heals and eventually reforms.